1. Field of the Invention
The present invention relates to a texture treatment process for the carbon substrate of a magnetic disk and for the carbon overcoat layer of a magnetic disk for preventing the generation of a stiction phenomenon between a magnetic head sliding surface and the magnetic disk surface.
2. Description of the Background
Prior art magnetic disks are generally constituted of an Al substrate coated with a Ni-P plating material or the like and a magnetic film formed thereon. In a magnetic disk reproducing unit, a magnetic head is disposed on a magnetic disk. The magnetic head, which flies by rotation of the magnetic disk, performs the writing and the reproduction; however, there often occurs a stiction phenomenon between the magnetic head sliding surface and the magnetic disk surface when the magnetic disk is at rest. The stiction phenomenon is due to the fact that, when the magnetic head sliding surface and the magnetic disk surface, being made extremely smooth, face each other and are spaced apart by a nano-gap, and the gap is filled with liquid lubricant molecules of O.sub.2, N.sub.2, H.sub.2 O and the like. This generates a large attractive force from the interfacial tension. The stiction phenomenon presents such a resistance to disk motion that the start-up of a motor for driving the magnetic disk consumes a large amount of power.
To prevent the stiction phenomenon, the mirror-finished surface of the substrate is textured to adjust the surface roughness prior to the coating of a magnetic film on the Al substrate of the magnetic disk. One of the texture treatments involves applying a moving polishing tape in the radial direction of a rotating Al substrate (coated with Ni-P plating material). Such a polishing tape comprises a tape on which abrasive made of silicon carbide, alumina or diamond are adhered. This mechanical texture treatment forms concentric scratches on the surface of the Al substrate of the magnetic disk. This process produces a rough surface in which the scratches are disposed in the circumferential direction.
The prior art texture treatment for an Al substrate of the magnetic disk is disadvantageous in that it is extremely difficult to suitably adjust the surface roughness. In order to increase the recording density, it is preferred that the flying height (spacing) of the magnetic head over the magnetic disk is minimized. However, as described above, the surface roughness of the Al substrate of a magnetic disk is a limiting factor. If the roughness is too large, the spacing between the magnetic head and the magnetic disk is too large, frustrating efforts to increase the recording density of the magnetic disk.
A carbon substrate has been proposed as a different type substrate for a magnetic disk in Kobe Technical Report, Vol. 39, No. 4, pp. 35 to 38, 1989. See also U.S. Pat. No. 4,716,078. The carbon substrate is lightweight, has high strength, and is excellent in heat resistance and surface accuracy. As compared with an Al substrate, the carbon substrate has the ability to improve the recording density of a magnetic disk.
The present inventors have earnestly studied the texture treatment of amorphous carbon substrates. As a result, it was found that, by the steps of polishing an amorphous carbon substrate to a specified surface roughness, and then heating it at a specified temperature in an oxidizing atmosphere, it is, possible to treat an amorphous carbon substrate to obtain a surface roughness suitable for a magnetic disk. Thus, the invention accomplished under this principle has been filed as Laid-open Japanese Patent Application Nos. 410434/1990 and No. 410436/1990. See also GB 2,242,423A.
In this process, the amorphous carbon substrate is polished to a specified surface roughness, and is heated at a temperature of from 300.degree. C. to 1000.degree. C., preferably, from 400.degree. C. to 700.degree. C. The oxidation reaction C+O.sub.2 .fwdarw.CO.sub.2 occurs. The gasified carbon (represented by the carbon dioxide gas) leaves surface irregularities on the polished surface. Accordingly, by control of the heating conditions such as the temperature and the treatment time, it is possible to easily treat the amorphous carbon substrate to a specified surface roughness, and hence to prevent the surface from being made more rough than necessary. This prevents the head stiction to the magnetic disk, improves the characteristics of the magnetic film formed on the amorphous carbon substrate.
The technique disclosed in the early applications and described above has achieved an effect not obtained by the prior art texture treatment processes. However, it has disadvantages. The above-described technique uses the chemical reaction between carbon and oxygen. In the chemical texturing process, the surface irregularities are determined by of crystal grains, aggregates of crystal grains and the polishing scratches. As the roughness of the texture is increased, the oxidation reaction at the polishing scratches is enhanced, often generating deeper scratches. The generation of the deeper scratches along the radial direction of the substrate tends to cause media errors such as bit shifts.